Electromechanical parking brakes are already known from the prior art which are preferably provided for braking a vehicle in the static operating state. In contrast to mechanical parking brakes, which are actuated by the driver using a hand brake lever and brake actuator connected thereto via a cable pull system and therefore bring about braking of a vehicle that is being parked, said electromechanical parking brakes can also be actuated automatically using an associated control unit, i.e. without the driver manually operating an operational control. This ensures, for example, that the electromechanical parking brake always generates a brake force that is high enough to keep the respective vehicle in the predefined parking position, in order, for example, to effectively prevent it rolling away.
In particular when parking a vehicle where specific ground or roadway conditions exist and, more precisely, for example in the case of a steep gradient and/or slippery roadway surface structure, it is necessary for secure braking or parking of the vehicle to be ensured by suitable automatic control of the electromechanical parking brake. One particular problem in this connection lies in the electromechanical parking brake taking over the braking or holding function of the electromechanical service brake.
Combined electromechanical brake systems are also known which are formed by an electromechanical service brake system with integrated parking brake function, i.e. a stationary vehicle is also braked via the electromechanical service brake and, more precisely, via its parking brake function. In contrast to braking the wheels of the rear axle in the case of a parking brake that is provided in addition to the service brake, all or selected vehicle wheels can be braked simultaneously by means of the described combined electromechanical service brake system with parking brake function. If, for example, a two-axle vehicle is on a gradient with a “μ split” and is to be securely parked there, with a combined electromechanical service brake system a braking moment can be generated on the front wheels in addition to a braking moment on the rear wheels.
A braking system is already known from DE 102 51 025 which comprises a service brake and a parking brake and in which the service brake force of the service brake applied to at least the rear wheels following activation of the parking brake is only released if the parking brake force applied to the rear wheels exceeds the generated service brake force. In such cases the parking brake force is typically only generated on the rear axle in the case of a two-axle vehicle with individual service brake units on the front and rear axles, i.e. the generated parking brake force acts solely on the rear wheels of the vehicle. A higher braking moment is required to hold the vehicle when it is parked on a gradient. To prevent the vehicle from rolling away a service brake force is simulated which only acts on the rear wheels. When the service brake force is inadequate the driver can increase the service brake force on the rear wheels generated via the service brake by actuating the brake pedal until the vehicle remains stationary. Only then is a parking brake force corresponding to the service brake force generated by the parking brake, so the service brake can finally be released at the rear wheels.
A method is also known from DE 103 51 026 in which a total parking brake force, which exceeds the total of the service brake forces applied to the wheels of a vehicle, is determined as a function of the vehicle's inclination. The brake force distribution is controlled via the braking system controller and, more precisely, the same total forces respectively are adjusted for this purpose. However, this assumes that the frictional forces between the respective wheel and the ground are high enough to be able to hold the vehicle stationary. As long as the vehicle is still being held by the service brake it is possible, in particular in the case of inhomogeneously distributed frictional conditions (“μ jump”), for the braking effect to be applied solely by the front wheels, because the rear wheels are on icy ground for example. With a brake force redistribution in accordance with the above pattern the total parking brake forces on the rear wheels, correctly determined for the vehicle inclination, would indeed be generated by means of the parking brake but are not sufficient to be able to securely hold the vehicle.
Methods for determining the maximum coefficient of friction on at least one wheel of a stationary vehicle are also known in which the vehicle is at a standstill on a gradient or incline and is equipped with a movement sensor. Stopping or movement of the wheel or of the vehicle is determined via the movement sensor. The wheel or vehicle is also fitted with an electrically controllable parking brake with which the brake force can be adjusted for the wheel to be braked. The service brake or the parking brake is continuously released in this connection until the vehicle has moved a short distance. As the brake is being released it is tested whether at least one wheel is being blocked, and a maximum coefficient of friction is determined for the blocked wheel.